1. Field of the Invention
The present invention relates to a process for producing hydrogen to efficiently produce hydrogen to be used for various industries such as petrochemical, fertilizer, iron manufacturing industries, etc., and an apparatus therefor.
2. Description of the Related Art
Hydrogen is widely used in main industries such as petrochemical, fertilizer, iron manufacturing industries, etc. Recently, hydrogen is practically used as clean energy for fuel of automobiles, fuel cells, etc.
However, there have been various processes developed for producing hydrogen. Among them, a first process for producing hydrogen by steam reforming hydrocarbon is mainly employed at present.
According to the first process, natural gas is mixed with steam, and the mixture is contacted with Ni catalyst at high temperature to obtain mixture gas of hydrogen, carbon monoxide, carbon dioxide, methane and steam when the first process is normally used in industries. Carbon monoxide is considerably contained in the mixed gas containing hydrogen obtained by the first process.
Since the carbon monoxide in the mixed gas can be, however, converted to hydrogen by means of shift conversion reaction, a process for converting the mixed gas into hydrogen by reactions of two stages of high and low temperature shift conversions is generally employed in the industries.
FIG. 3A shows a configuration of a system for carrying out the first process for producing hydrogen. In FIG. 3A, a high temperature shift converter or reactor 1a contains a high temperature shift conversion catalyst 2a therein so as to carry out a high temperature shift conversion reaction, and is thermally insulated from the exterior. Reforming gas 3 obtained by steam reforming hydrocarbon is introduced into the reactor 1a. The reforming gas 3 containing carbon monoxide is fed to a cooler 5 as the gas 4 after a part of carbon monoxide in the reforming gas 3 is converted into hydrogen and carbon dioxide in the high temperature shift converter or reactor 1a. The gas 4 is cooled by the cooler 5. The cooled gas 6 is introduced into a low temperature shift converter or reactor 7. A low temperature shift conversion catalyst 8 is contained in the low temperature shift converter or reactor 7. Thus, the cooled gas 6 introduced from the cooler 5 is contacted with the low temperature shift conversion catalyst 8 and approx. 90% of the carbon monoxide in the cooling gas 6 is converted into hydrogen and carbon dioxide to obtain the gas 9 in the low temperature shift converter 7.
Unreacted carbon monoxide remains in the gas 9 containing hydrogen. Since the carbon monoxide acts detrimentally in some application of the hydrogen, the carbon monoxide content must be practically lowered to concentration of a low level which does not affect a detrimental action. Therefore, a highly active catalyst is employed under the conditions of 200.degree. to 250.degree. C. in the commercial low temperature shift converters.
FIG. 3B shows the amounts of gases to be produced in various sections of the system in experiments in FIG. 3A. In FIG. 3B, numerals 3, 4, 9 and 6 in upper columns respectively correspond to the gases produced in the sections in FIG. 3A.
In order to lower the concentration of the carbon monoxide in the reforming gas from 6.5 to 0.2%, the following reactions are required. The carbon monoxide in the reforming gas is first lowered from 6.5 to 2.2% in the high temperature shift converter or reactor 1a the temperature raised by the reaction heat in this case is then lowered to 205.degree. C., and 90% of the content of the residual carbon monoxide in the reforming gas must be further converted in the low temperature shift converter or reactor 7. Hereafter the term reactor in shift converter or reactor is omitted for abbreviation. And the term reactor is also used for converter.
The reason why the concentration of the carbon monoxide in the reforming gas is reduced by the two stages of the shift converters 1a and 7 as described above is because the temperature of the outlet of the reactor is raised due to the heat generated according to the conversion reaction if it is carried out in one stage of a shift converter and the concentration of the carbon monoxide cannot be lowered to a low level at this temperature level due to chemical equilibrium.
However, in order to convert the carbon monoxide in the reforming gas by the two stages of high and low temperature shift converters, shift converters must be designed to fill a high temperature shift conversion catalyst 2a in the high temperature shift converters 1a of the former stage and to decrease the outlet temperature of the shift converter within the range of the heat resistant temperature of the catalyst. Further, the shift converters must also be designed to decrease the outlet temperature of the low temperature shift converter 7 of the latter stage within the heat resistant temperature of the low temperature shift conversion catalyst 8 filled in the shift converter 7.
Therefore, it is necessary to cool the heat generated in the shift conversion reaction so as to carry out the shift conversion reactions of the two stages by one stage.
FIG. 4A shows a configuration of a system for converting to hydrogen by shift conversion reaction of one stage. In FIG. 4A, reforming gas 3 reformed from hydrocarbon by steam is introduced to a cooler 11. The temperature of reforming gas 3 is lowered to 205.degree. C. by the cooler 11 to obtain the reforming gas 12a, and the reforming gas 12a is then introduced into a low temperature shift converter 13. The low temperature shift converter 13 contains a catalyst 14 for producing hydrogen from the gas 12a by shift conversion. The shift converter 13 has a cooler 15 for removing the reaction heat generated during the shift conversion reaction. The gas 9 is produced from the cooled gas 12a which contains carbon monoxide by converting most of the carbon monoxide into hydrogen and carbon dioxide by shift conversion reaction in the low temperature shift converter 13.
FIG. 4B shows the measured result of the quantity of gas cooled to 205.degree. C. by the cooler 11 in the configuration of the system.
However, since the cooler 15 for removing reaction heat is provided in the low temperature shift converter 13 of the system for converting at the one stage, its facility is complicated, its facility cost becomes expensive and is disadvantageous in its economy.
In the conventional system for converting to hydrogen by the shift conversion reactions of two stages, the shift converter outlet temperature of the former stage in which high temperature shift conversion catalyst is filled must be designed to fall within the range of the heat resistant temperature of the catalyst, and the outlet temperature of the low temperature shift converter of the rear stage in which the low temperature shift conversion catalyst is filled must be designed to fall within the range of the heat resistant temperature of the catalyst.
In the conventional system for converting carbon monoxide to hydrogen by shift conversion reaction of one stage, the cooler 15 is provided to remove reaction heat in the low temperature shift converter 13. Therefore, the facility itself is complicated, its facility cost becomes expensive, and is disadvantageous in economy.
Further, there is, in addition to the above-described method, a conventional process for introducing water directly into the shift converter to suppress the temperature increase of the reaction gas by vaporization and to provide an effect of accelerating the shift conversion reaction. However, according to this method, its system is complicated, the introduced water must be eventually recovered, and negative points of view exist in thermal energy.